The present invention relates to a method of reproducing pictures by fast forward (F.F.) and fast backward (F.B.) reproduction on the basis of moving picture data coded at high efficiency. More specifically, the present invention relates to a method of reproducing pictures by selectively reproducing only picture frame data compressed in accordance with intra-frame method from moving picture data coded at a high efficiency in F.F. and F.B. reproduction. Here, the moving picture data consists of picture frames whose picture data are compressed in accordance with intra-frame method and picture frames whose picture data are compressed in accordance with inter-frame prediction method.
Various high efficiency compression systems for compressing moving picture signals at a high efficiency before transmitting, recording and reproducing the moving picture signals have been widely so far researched and proposed. As one of these systems, there has been proposed a high efficiency compression coding system. Picture data compressed by this system consists of picture frames whose picture data are compressed in accordance with intra-frame method and picture frames whose picture data are compressed in accordance with inter-frame prediction method.
Further, it has been already tried to record moving picture data compressed at a high efficiency on a small-sized disk. On the other hand, MPEG (moving pictures expert group) has proposed various data formats related to the high efficiency compressed picture data in sequence. This proposition is to prepare an international standard related to the systems of compressing picture data by coding moving video signals at a high efficiency. Accordingly, there have been widely researched and developed various apparatus for transmitting, recording and reproducing moving picture data compressed at a high efficiency in accordance with the data formats proposed by the MPEG, that is, moving picture data compressed at a high efficiency in accordance with the MPEG system.
On the other hand, in the field of the moving picture data coding system (MPEG system) for media (e.g., CD-ROM) for recording digital data, moving video signals are now compressed at a high efficiency by adopting a prediction coding method, in the same way as with the case of the high efficiency coding system discussed above.
As the prediction method, in this case, three sorts of picture modes are adopted as follows: intra-picture frame (referred to as I frame, hereinafter) by which picture data are compressed at a high efficiency on the basis of the intra-frame coding method; predicted-picture frame (referred to as P frame, hereinafter) by which video signals are compressed at a high efficiency on the basis of the inter-frame prediction coding method using a past frame picture data; and bi-directional prediction picture frame (referred to as B frame, hereinafter) by which video signals are compressed at a high efficiency on the basis of the inter-frame prediction coding method using both past and future frame picture data.
Further, a header is attached to the digital picture data whose frames are arranged in a predetermined arrangement mode on the time axis, in order to obtain the picture coded data.
In the above-mentioned MPEG system, since CCIR (ITU-R) has been also standardized, the MPEG system will be explained hereinbelow as an example of the high efficiency compression systems for moving picture signals having I, P, and B frames.
Now, in the MPEG system, the mutual relationship among the picture data compression ratio CI of I frames, the picture data compression ratio CP of P frames, and the picture data compression ratio CB of B frames is determined as CI&lt;CP&lt;CB. Further, the picture reproduction must be started beginning from a sequence header as an entry point. Further, in the case of B-frame reproduction (picture data are predicted on the basis of picture data of past and future frames), the picture data of the future P frame used for prediction of the picture data of a B frame must be recorded in front of the B frame.
A picture reproducing apparatus follows the steps shown in FIG. 1 to reproduce pictures by F.F. or F.B. reproduction on the basis of the moving picture data coded at high efficiency in accordance with the MPEG system. In practice, whenever data indicative of the F.F. or F.B. reproduction mode is entered, a central arithmetic processing unit (CPU) of the picture reproducing apparatus executes the sequential operations, in accordance with the respective steps as shown by the step S5 and after in FIG. 1.
FIG. 1 shows the procedure of F.F. or F.B. reproduction of pictures on the basis of the moving picture data coded at high efficiency by the MPEG system. When the procedure of FIG. 1 starts, in response to a command of CPU, a disk drive moves an optical head for seek operation. Data read by the optical head are once stored in a buffer memory. The stored data are read out to supply a bit stream to an MPEG video decoder. The MPEG video decoder detects and decodes I frames by checking the whole supplied bit stream. Whenever the MPEG video decoder completes one I frame decoding operation, the completion of the operation is informed to the CPU. Then, the CPU gives a command to the disk drive unit in such a way that the optical head can seek a position at which the succeeding I frame may exists.
Therefore, whenever picture data of the succeeding I frames are reproduced, a sum total time of the three periods of time as follows is required: (1) a period of time required to skip to a bit stream position at which the succeeding picture reproduction is to be started; (2) a period of time required to detect an I frame after the bit stream reproduction has been started from the skipped position; and (3) a period of time required to reproduce the detected total I frame. In this case, however, intervals between the succeeding I frames are not constant but different each other in the bit stream with I, P, and B frames.
Therefore, the skipped position of the bit stream at which the succeeding picture reproduction is to be started may be determined at a position far enough ahead from the position at which an I frame is supposed to exist. The I frame is detected after the bit stream reproduction has been started from the skipped position. As a result, a time required to detect the I frame may be increased, so that the number of reproduced pictures per predetermined period of time is reduced and thereby the motion of the reproduced picture is not smoothed sufficiently. This problem arises in any high efficiency compression systems for moving picture signals with I, P, and B frames in the same way as with the case of the above-mentioned MPEG system.
To overcome this problem, the applicant has already proposed such a method of reproducing pictures by F.F. and F.B. reproduction from high efficiency coded moving picture data in Japanese Patent Laid-Open No. 6(1994)-276485.
In this method, I frames are previously arranged in the bit stream in such a way that an averaged interval between the I frames becomes a previously determined constant value. This I frame arrangement is good to select only I frame pictures from the high efficiency coded moving picture data in order to display I frame pictures at a predetermined constant interval.
In the case of F.F. reproduction, the succeeding I frame to be reproduced after an I frame now being reproduced is searched beginning from a position obtained by subtracting a constant value K from an integer-time position of the predetermined average I frame interval.
On the other hand, in the case of F.B. reproduction, the succeeding I frame to be reproduced after the I frame now being reproduced is searched beginning from a position obtained by adding a constant value M to an integer-time position of the previously determined I frame average interval.
This reproducing method in order to shorten the detection time of I frames in F.F. and F.B. reproduction from a storage medium, bit streams stored thereon being constructed in such a way that the average interval between I frames becomes a predetermined interval value.
However, in this method, such an advantage cannot be expected when the average interval between I frames in the bit stream recorded in the storage medium is different from the predetermined average value or there is a variety of average intervals.